1. Field of the Invention
The present invention relates to a hydro-mechanical transmission having a hydrostatic unit and in particular to a method and means for determining the actual output speed of the hydrostatic unit.
2. Description of Related Art
The controller of a hydro-mechanical transmission commands a given output speed for the hydrostatic unit based on a variety of input factors. For proper control it is necessary to have a feedback to the controller of the actual output speed of the hydrostatic unit. This has been provided in the past by the use of a directional Hall effect sensor and a magnetic ring carried on the hydro unit output shaft or on a gear driven by the hydro unit output shaft. One drawback of this approach is that the directional Hall effect sensors and the magnetic ring are relatively expensive. Another drawback is that when the hydrostatic unit is at near zero stroke, the hydro unit output speed is low and the frequency of the speed pulses is also low. Therefore the update rate for measuring the hydro unit output speed is slow.
It is therefore an object of the present invention to overcome the above drawbacks associated with the use of a directional Hall effect speed sensor.
The transmission of the present invention uses two, non-directional, relatively inexpensive variable reluctance speed sensors on different gears in the transmission to accurately determine the speed and direction of the hydrostatic unit output. One sensor is on the hydro unit output of the hydrostatic unit and measures the hydro unit output speed directly. The other sensor is on the transmission planetary carrier and measures the hydro unit output speed indirectly. The hydro unit output speed must be calculated based on the current engine speed and the known transmission gear ratios. When the hydrostatic unit is stopped or turning at a slow speed, the carrier is turning at a relatively fast speed. Furthermore, when the carrier is stopped or turning at a slow speed, the hydrostatic unit output shaft is turning at a relatively fast speed. The sensor that is measuring the faster actual speed is the sensor that is used to determine the hydro unit output speed. There is a transition period during which the two measured speeds are relatively close to one another. In the transition period, the hydro unit output speeds determined by the two sensors are averaged and the average value used by the controller as the hydro unit output speed.
By using the sensor with the highest actual speed measurement, the frequency of the speed measurement is always greater than or equal to 250 Hz. As a result, a new and accurate speed can be measured every 4 ms for closed loop speed control. The variable reluctance sensors are inexpensive and can measure existing gears in the transmission without the need for a pressed-on magnetic ring. Furthermore, having two sensors to measure the hydrostatic unit speed allows for error checking and onboard diagnostics by the controller.